Yeah, we can only speculate. I've also been puzzling about what they are doing. The best guess I can come up with is that, maybe at the start, the Russians fell victim to their own propaganda, and went in believing that they only needed a small force because the Ukrainians would welcome them with open arms and readily topple their supposedly drunken, drug-ridden, Nazi, Government. And now the Russians have discovered that was a fantasy, they're stuck with not having arranged the logistics to send in many more troops. There are also some reports that the Ukrainians have been cunning and targeted Russian logistics behind the main front too - if those reports are true, that may have helped a lot.
I also wonder whether Russian military equipment is possibly in much worse condition than we imagined (wouldn't be surprising, given how corrupt Russia is), and the stuff they've sent in is in fact the only stuff that is really in fight-worthy condition, with the remainder being left behind to threaten but they don't want to risk using it in actual battles.
That's all total speculation and I could be totally wrong, but it's my best guess so far.